1. Field of the Invention
The present invention relates to a technique of purifying NO.sub.x -containing exhaust gases by use of catalyst for purifying NO.sub.x -containing exhaust gases comprising a zeolite material in the presence of ammonium acetate as a reducing agent.
2. Background Art
The incineration of industrial wastes and domestic refuse forms NO.sub.x, CO, SOx, hydrochloric acid and offensive smells according to the origin, kind and composition of these wastes. Because of this, various measures have been taken to treat exhaust gases containing these irritants and further research and development has been conducted and the same may be said of exhaust gases exhausted from cars, thermal-power generation systems and co-generation systems employing a gas engine.
Particularly, regarding the treatment of NOx (nitrogen oxides) in various exhaust gases, various methods have been known as so-called smoke-exhaustion denitration techniques including non-catalytic reduction, catalytic cracking, non-selective or selective catalytic reducton, adsorption, electron rays irradiation, molten salt absorption and reduction absorption. Of the above, catalytic reduction performing purification using a catalyst in the treatment usually converts NO.sub.x to N.sub.2 to make it harmless and hence has been noted particularly. As catalysts used for the catalytic reduction, noble metals such as Pt, Rh and Pd, metal oxides such as TiO.sub.2, V.sub.2 O.sub.5, Cr.sub.2 O.sub.3 and Fe.sub.2 O.sub.3, rare earth metal oxides and sulfides, and zeolite-based catalysts have been used.
Zeolites are used effectively as a catalyst itself and also as a carrier for various catalysts. Zeolites, in general, are characterized by comprising aluminosilicate in composition, being crystal line and having a three-dimensional network, and containing cations with high ion-exchange properties. There are a lot of kinds thereof according to the ratio of an alumina (Al.sub.2 O.sub.3) component and a silica (SiO.sub.2) component constituting it and the crystalline constitution thereof, and typical examples thereof include analcime, chabazite, mordenite, faujasite and clinoptilolite.
In addition to natural zeolites, synthetic zeolites are produced according to hydrothermal synthesis and are on the market. They are used broadly, for example, as fillers for paper, moisture-absorbing agents, ion exchangers, molecular sieves, catalysts for various reactions and carriers for catalysts, and denaturation, modification and other improvements thereof have been performed successively in accordance with various uses and reactions thereof. In the field of catalysts used for purifying NO.sub.x -containing exhaust gases, it has been proposed to deposit a metal such as copper and iron on zeolites for some improvement in the purification efficiency.
For example, the official gazette of Japanese Laid-Open Patent Publication No. 2-194819/1990 shows a method of reducing nitrogen oxides present in an exhaust gas to nitrogen by mixing ammonia with said exhaust gas, and introducing the resultant mixture onto an acid-resistant zeolite catalyst containing Cu and/or Fe. However, it describes that the techniques of allowing the metals to be contained in the zeolite may be ion exchange, impregnation, or precipitation or mixing of metal oxides or metal salts with zeolites, and calcination in an oxidization or reduction atmosphere, or ammonia-containing atmosphere. Therefore, metals such as Fe are only contained in the ready-made zeolites by the aftertreatment and not contained in zeolites during the synthesis thereof.
Also, the official gazette of Japanese Laid-Open Patent Publication No. 2-4453/1990 proposes a method of producing a denitrating catalyst (namely, zeolite with a transition metal deposited thereon) for subjecting nitrogen oxides in an exhaust gas to catalytic reduction in the presence of ammonia. Here, zeolite catalysts are produced by using Al, Si, compounds of alkali metals or alkali earth metals and salts of transition metals as materials, adjusting molar ratios of the material components to predetermined ratios and subjecting a gel solution with an effective alakli concentration adjusted to a hydrothermal treatment. Transition metals, as raw materials, are specifically Cu, Fe, Ni, Co and V and the zeolites to be obtained here are mordenite and a pentasil-type zeolite.
In the above-mentioned catalytic reduction denitration, purifying NO.sub.x in a NO.sub.x -containing exhaust gas by means of a catalyst, a reducing agent is indispensable. As reducing agents, ammonia, hydrogen, methane and other hydrocarbons, and carbon monoxide can be used. Of these agents, since ammonia is excellent in selective reactivity to NO, it is now adopted for practical use in denitration. The above official gazette of Japanese Laid-Open Patent Publication No. 2-194819/1990 and the official gazette of Japanese Laid-Open Patent Publication No. 2-4453/1990 make it essential to use ammonia as a reducing agent.
In this point, the official gazette of Japanese Laid-Open Patent Publication No. 2-203923/1990 points out problems such as an ammonia leak, when using ammonia as a reducing agent similarly to the case of the above art, and proposes the use of ammonium salts or amine compounds instead of ammonia. Ammonium salts in the proposal are ammonium carbonate, ammonium bicarbonate, ammonium formate and ammonium acetate, and the amino compound is urea. On the other hand, as catalysts in the proposal, known ones such as a carrier-depositing type, a non-carrier-depositing type and a Raney type can be used, and anatase-type titania with a metallic oxide such as V.sub.2 O.sub.5 deposited thereon exhibits particularly excellent denitration performance. However, there is no description regarding a zeolite catalyst and only this V.sub.2 O.sub.5 catalyst is described specifically in the proposal.
Also, the official gazette of Japanese Laid-Open Patent Publication No. 2-203923/1990 discloses, in the examples thereof, reaction ratios when introducing aqueous solutions of the above-exemplified reducing agents into a reaction tube, using "anatase-type titania with V.sub.2 O.sub.5 deposited thereon (deposition ratio of vanadium: 4 weight %)" as a catalyst at a reaction temperature within the range of 200.degree. to 450.degree. C. to an exhaust gas containing NO of 100 ppm prepared for an experiment. According to the above, when using ammonium acetate as a reducing agent, almost the same effect as when using ammonia described as Comparative Example there is obtained.
However, these reaction ratios do not exceed the equivalent reaction of NO and a nitrogen atom in an ammonium salt.
That is, when using ammonia as a reducing agent in catalytic reduction denitration and, as described above, also when using ammonium salts such as ammonium carbonate, ammonium acetate, ammonium bicarbonate and ammonium formate, the reaction between a N atom in the reducing agent and NO in the exhaust gas is below the equivalent, and hence NO.sub.x (NO) beyond the amount of the reducing agent (in terms of N) added cannot be removed.
On the other hand, when treating an exhaust gas by means of a solid catalyst, the catalyst is employed 1 in a form of a fluidized bed or a fixed bed as a particle, 2 in a form of a fixed bed as a pellet, or 3 in a form of a monolith deposited on a monolithic carrier. Generally, they are selected according to origin and kind of NO.sub.x -containing exhaust gases and other various requisites. However, there is no instance of showing that the form of a monolith in the above 3 improves performance as compared with the form of the above 1 or 2.